Canned heat products, most notably Sterno, a trademarked product consisting of a gelled alcohol mixture, are common. The products roots lie in antiquity as many candle and lamp heaters existed back to the shell shaped lamps of prehistory burning fats and vegetable oils.
The most recognizable product in the category of portable heating sources is a can of sterno, a popular alcohol based source of flames that consists of a gelled mix including a high proportion of alcohols which are easy to ignite and burn readily.
The Sterno product produces a sheet of flame across the surface of the container that holds the flammable gel, and this sheet flame, unlike a wicked candle, provides a large flame area and a correspondingly large heat output.
The increasingly restrictive rules on shipment of flammables has created a need for products that can burn like the Sterno product, but which are safe to ship. This means that the product must have a high flash point.
The use of portable fuel cans such as the above mentioned Sterno product is widespread for campfire replacement, for buffet warming duty, for hot service pans, fondue heating and innumerable other applications where localized heat or warming is needed. The Sterno type cans, since they are useful and have long been desirable in consumer and food service markets have been the subject of a number of prior patents. Examples of portable burners are U.S. Pat. No. 4,725,225 (Grabitt); U.S. Pat. No. 3,042,108 and U.S. Pat. No. 3,290,907 Boij et al); U.S. Pat. No. 4,624,633 (Bandel); and U.S. Pat. No. 3,516,774 (Livingston). These designs are all wick based and use either a true wick or other materials that act as a wick to provide a hot surface that vaporizes the barely burnable materials making them easily ignited.
As noted in comments on candles above, even with large bundles of fibers acting as a wick, the burning area is severely constrained by the wick size. A Can width wick would deplete the fuel reservoir very quickly and defeat the need for long lasting heating. Wicks are a problem and there is a need for a sheet of flame over the surface of a container that wicked devices in prior patents cannot create.
The wick also typically is used not as in candles but with a liquid fuel that is readily wicked up the wick to the flame front area. When liquid fuels are used, the container seal is critical in shipping since there is a possibility of fluid leakage, and the possibility of accidents due to spill of contents during use is a real problem. There is a need for a liquid fuel storage and transport method and a device that prevents spillage of liquid fuels.
The solution to the wick type problems have been variations on U.S. Pat. No. 4,850,858 (Blankenship et al) where a liquid fuel was turned into a gel which would support flame across its entire surface without a wick and still was not fully fluid. As heat is applied, the gel melts and becomes fluid and is still prone to spillage. In addition the gel components being a small part of the total composition do little to make the alcohol rich gel shippable. If the retardant elements or non-burnable elements in the gel were increased to the point where the gel was safe to ship and spill resistant, the flame would be sufficiently reduced to render the heater ineffective. There is a need for a gel like material that is sufficiently flammable and safe to ship.
The gel type burners also create other safety problems. They can flash or explode as they are ignited due to vapor above the surface of the gel layer, and they burn down causing eventual heating of the can, and persons touching the can of partly burned, liquified gel may react to this hot surface and drop the container thus causing fires of other problems. Safety requires a better flame type container for heating purposes.
Another way around the Sterno type gel problem is to find a material that is flammable enough to burn in sheet fashion across the entire surface of a container but which is a semisolid or at least viscous enough to have a flash point that is acceptable for shipping. Generally the problem of gasification of the viscous material without a wick or extended surface prevents sheet burning of such materials. U.S. Patent applications have been filed for specific materials, the manufacture of units in the Bahamas and sale in the United States in 1989 and 1990 of such products as are covered in the Levinson Patent (U.S. Pat. No. 5,193,521). These early manufactures used both blends and nonblended glycol materials with mineral wool or glass fiber fillers.
One application by Levinson was for a particular blend of 16 parts of diethylene glycol and one part of isoparaffin that partially cures this problem. The presence of over 94% of the viscous material and only around 6% of the much more flammable isoparaffin which is subject to restricted shipping and is normally a liquid lowers the flash point of the mixture sufficiently to allow shipping of the mixture without restrictions. Despite the lowered flammability, this mixture does burn in sheet manner and thus does provide a satisfactory heat source when in a container. The inventor of this mixture found that the diethylene glycol by itself could not be ignited without the presence of an intermixture of the flammable liquid. Unfortunately the diethyleglycol and the isoparaffin materials were non-miscible and thus separated to form separate layers which defeated the purpose of the invention.
Common uses for canned heat products are as heaters in chafing dishes and food service applications in hotels and food service and buffets as well as in camping, backpacking and wilderness uses where compact heaters and stoves are required.
One very common product that illustrates the complexity of canned heat products and the problems they have is shown in U.S. Pat. No. 5,307,799 to Scarnato et al. This product uses a fuel, diethylene glycol, which is hard to ignite. There were a series of approaches to making this product ignitable, one involving the mixing of two fluids, one at lower concentration which is easier to ignite and a second, consisting of a majority of the fuel, which was less ignitable. This concept was only partly successful and later a concept of a pool of more flammable material wicked through a bed of fibrous material impregnated with less flammable fluids was patented. This Patent requires a wick, a bed of fiber, and separate addition in succession of two different measured alloquats of fluids. The resulting manufacture is difficult.
The reason that one material wicks well and another does not is poorly understood. The concept of wicking requires wetting of a surface, a fluid that is relatively low viscosity, a group of surfaces that are near each other or in partial (but not full) contact, and the ability of the fluid to form a meniscus (wet a surface) and also seems to relate-to the roughness of the surface. The ignition of the surface to form a flat film flame at the surface is also poorly understood but it requires at least the foregoing factors and others not defined.